Optical communication networks may carry both digital and analog transmissions. Digital transmission is more commonly used simply because it is the more widely known and understood method of transmission. It is used extensively for wavelength division multiplexing (WDM) and time division multiplexing (TDM). However, there are several applications where a signal to be transmitted from one point to another cannot conveniently and economically be converted to digital form. For example, in telephone transmissions, an FDM (frequency division multiplexing) master group of 600 4-kHz voice channels is stacked in frequency from 60 to 2788 kHz. While such a signal could be sampled and encoded into a binary signal, the linearity and fidelity requirements on the coding and decoding in order to prevent cross-talk between the channels are extremely restrictive. Other applicatiors where analog transmission is preferred are video and CATV signals where the cost of digital coders and decoders are prohibitive.
In order to detect an analog signal the detector must be relatively sensitive, having a good signal-to-noise ratio (SNR). The SNR for the receiver decreases as the receiver bandwidth increases. In order to accommodate a large number of signals a large bandwidth is required, thus decreasing SNR and increasing the total optical power received, creating a risk of detector saturation and nonlinear distortion. Further, the probability of error increases due to the interference caused by vector addition of various signals at the detector.
It is known that optical bandpass filters can be fabricated from optical fibers (U.S. Pat. No. 4,577,924, commonly owned and issued to this Applicant) and from planar waveguides (U.S. Pat. No. 5,002,352, commonly owned of which this Applicant is a co-inventor). Optical bandpass filters so fabricated create a resonant cavity which is resonant at a desired frequency and, thus, reduces or filters out other frequencies. Such filters are relatively easy to fabricate and provide other benefits including reduced size, weight and cost, and reduced sensitivity to electromagnetic interference as compared with other types of filters.
It would be desirable to provide a means for controlling FDM optical signals which would permit an increase in the bandwidth to accommodate more analog signals while improving the SNR of the detector to provide greater sensitivity and low error rate. It would be further desirable to provide such a filtering means using relatively simple devices which are readily fabricated. It is an object of the present invention to provide such means.